1. Field of the Invention
This invention pertains generally to the field of steering systems for snowmobiles, and particularly to a snowmobile power steering system.
2. Description of the Prior Art
Various types of steering mechanisms have been designed and are currently used for snowmobiles. The systems generally include the steering column and a linkage arrangement which transmits forces from the steering column to the skis. One such steering system is shown in U.S. Pat. No. 3,550,906 to Watkins. Snowmobile steering systems have been developed in which the steering is cushioned such as those disclosed in U.S. Pat. Nos. 4,509,766 to Yasui et al., No. 4,374,591 to Kobayashi, and No. 4,352,501 to Takagi. These cushioned steering systems generally include a shock absorbing means which dampens the telescopic movement of portions of the steering mechanism. The effect is to allow smoother steering in typical steering linkage systems.
Many power steering systems have been developed for motor vehicles. The U.S. Pat. No. 4,003,404 to Burckhardt discloses a servo-steering system for vehicles utilizing a servo-control valve which responds to motion of a steering nut movable on the steering spindle to control the steering adjuster. The U.S. Pat. No. 4,440,254 to Shibahata et al. discloses a compliance steering control system for four-wheeled vehicles which directs pressure to a hydraulic servo to help turn the front wheels in response to the rotation of the steering wheel, and which also tends to turn the pair of rear wheels in the same direction that the front pair of wheels is turned in response to the rotation of the steering wheel. A hydraulic control unit for a power steering assembly is disclosed in U.S. Pat. No. 4,462,209 to Thomsen et al. The unit has a metering motor and steering wheel actuated valves for directing metered pressurized fluid to one selected port of a steering servo-motor and withdrawing fluid from the other port of the servo-motor to a reservoir. The unit utilizes a particular regulating valve for directing pressurized fluid to other consumer units when the steering control is in neutral. A hydraulic control system for off-highway self-propelled work machines is disclosed in U.S. Pat. No. 4,517,800 to Karakama et al. This control system has two fixed-displacement pumps for supplying pressurized fluid to a pair of steering cylinders via a steering valve and to a pair of implement cylinders via an implement valve. This system is generally used with large off-road work machines such as graders and dump trucks.
Snowmobile skis tend to develop an oscillating motion which is dependent on the length of the ski in contact with the ground surface. This oscillating motion varies with speed and is transmitted through the steering linkage. If this oscillating motion is transferred to the control valve of a power steering system, the power steering will develop a chatter. In the prior art, either a coil spring or a torsion bar was used to control the deflection of the control valve. Both coil springs and torsion bars have natural frequencies of their own. Therefore, when another frequency is introduced to these parts, they may either magnify or absorb the oscillating motion. If the coil spring or torsion bar magnifies the oscillating motion, the chatter within the steering mechanism may render the snowmobile uncontrollable.